Probably won't ever come up unless you try to take them on duty overnight, or if you are drug tested and either can't produce the Rx, or the blood level is too high.
As long as it's not a BS prescription for something like marijuana, or some at places, actual narcotics, you should be good, so long as it doesn't effect your ability to work.
Don't take it within 8-12hrs (each employer should have a policy) before a shift, don't take it at work and have a valid script come drug test time and you should be fine.